Casting steel in the form of conventional ingots involves high costs for processing the ingots to billets. Moreover, since killed steels are cast as ingots with a sinkhead, there is a yield loss of 10 percent to 20 percent when the sinkhead is removed in processing. Accordingly, to avoid the high-processing costs and large yield losses which are connected with processing conventional ingots, continuous steel casting has been increasingly used. However, high-alloy steels, such as the steel used in tools, and ball bearing steels, are not suitable for continuous casting because of the segregates which form when steels having large solidification intervals are cast continuously.
The forming of segregates in continuous casting occurs because of the shape of the solidification front. More particularly, during the solidification interval, a solidified shell is formed, and on the inside of the shell, a semi-solidified structure of dendrites is formed interspersed with residual melt. The interior of the shell shrinks during the solidification process, and the shrinkage is compensated by residual melt being pressed into the interior of the shell. This causes stresses to occur in the dentritic network and it finally ruptures. The ruptures or cracks are then filled with residual melt and segregates are formed. Segregates can be avoided if the continuous casting process is carried out extremely slowly so that the solidification front has a favorable shape. However, such a process is uneconomical because of the very slow casting rates.
Broadly, it is an object of the present invention to provide an improved method for casting steel ingots which overcomes the aforesaid problems. Specifically, it is within the contemplation of the present invention to provide an improved method which causes the upper surface of the ingot to solidify before the interior of the ingot is solidified and maintains the upper surface of the ingot at a sufficiently high temperature so that it may deform plastically and sink downwards when the interior of the ingot solidifies and shrinks.
It is a further object of the present invention to provide an improved method for casting steel ingots in molds in which the ingots have substantially uniform chemical composition throughut their cross section and a substantially uniform distribution of segregates throughout their cross section, rather than being concentrated at the upper surface of the ingot.